The Collaborative International Dictionary of English v.0.48:

Jactancy \Jac"tan*cy\ (j[a^]k"tan*s[y^]), n. [L. jactantia, fr.
   jactans, p. pr. of jactare to throw, boast, freq. fr. jacere
   to throw; cf. F. jactance.]
   A boasting; a bragging. [Obs.]
   [1913 Webster]